Nitrous oxide is a powerful greenhouse gas. Estuaries are potential sources of nitrous oxide, but their emission estimates are associated with significant uncertainties due to limited data availability and high spatiotemporal variability. In this study, we present nitrous oxide measurements from nine research cruises done from 2017 to 2022 along the Elbe estuary (Germany), a known source of nitrous oxide. We calculated nitrous oxide saturation, flux densities and emissions. Further, we analyzed nutrient and oxygen concentrations to identify nitrous oxide production pathways and drivers. We found that the estuary was a year-round source of N₂O, with highest emissions in winter when dissolved inorganic nitrogen loads and wind speeds are high. However, in spring and summer, nitrous oxide saturation and emission do not decrease in scale with dissolved inorganic nitrogen loads indicating that in-situ production. Two hot-spots of nitrous oxide production existed: the Hamburg Port and the mesohaline estuary downstream of the estuarine turbidity maximum zone (MTZ). Nitrous oxide production was fueled either by riverine organic matter in the Hamburg Port or by marine organic matter in the MTZ. Previous measurements in the Elbe Estuary revealed that nitrous oxide saturation was decoupled from DIN loads after an improvement of water quality in the 1990s that allowed for phytoplankton growth to reestablish, highlighting the overarching control of organic matter on nitrous oxide production in the well-mixed temperate Elbe Estuary.

Primary Presenter: Gesa Schulz, Helmholtz Centre Hereon (gesa.schulz@hereon.de)

Authors:

Tina Sanders, Helmholtz-Zentrum Hereon  (tina.sanders@hereon.de)

Yoana Voynova, Helmholtz-Zentrum Hereon  (Yoana.Voynova@hereon.de)

Hermann Bange, GEOMAR Helmholtz Centre for Ocean Research Kiel  (hbange@geomar.de)

Kirstin Dähnke, Helmholtz-Zentrum Hereon  (Kirstin.daehnke@hereon.de)